In the Universal Mobile Telecommunications System (UMTS) system, a User Equipment (UE) with a Radio Resource Control (RRC) Connection can be in CELL_DCH, CELL_FACH, CELL_PCH or URA_PCH state. UE with data traffic is put into CELL_DCH or CELL_FACH where it is able to transmit and receive user data. CELL_FACH state is usually used for UE with low burst traffic activity.
Enhanced CELL_FACH in UMTS allows the UE to receive HSDPA (High Speed Downlink Packet Access) packets. This enables the UE to receive large burst of downlink data. Enhanced Uplink CELL_FACH in UMTS allows the UE to send a large burst of uplink data using HSUPA (High Speed Uplink Packet Access). The Enhanced Uplink CELL_FACH is a superset of Enhanced CELL_FACH, meaning the HSUPA in CELL_FACH requires the HSDPA in CELL_FACH.
The bursty nature of smartphone traffic is suited for the Enhanced Uplink CELL_FACH state since it uses the resources (HSDPA and HSUPA resources) more efficiently compared to that in CELL_DCH. The number of smartphone devices is expected to increase significantly in the future and hence it is expected that more UE will reside in the Enhanced Uplink CELL_FACH state.
MAC-i and MAC-is entities are two of the building blocks for Enhanced Uplink CELL_FACH transmission. The MAC-i implements the Hybrid Automatic Repeat Request (HARQ) protocol, which is based on synchronous (re)transmission in the uplink and synchronous downlink Acknowledge/Negative-acknowledge (ACK/NACK). The number of HARQ process depends on the Transmission Time Interval (TTI): 8 processes for 2 ms TTI, and 4 processes for 10 ms TTI. The parameter “Maximum Number of HARQ retransmissions” (MaxNHarq) defines the upper limit on the number of HARQ retransmission.
The MAC-is at the Radio Network Control (RNC) implements the re-ordering and reassembly functions. The re-ordering process is per logical channel and is based on the Transmission Sequence Number (TSN). At every TTI, all data belonging to the same logical channel is carried in a MAC-is Protocol Data Unit (PDU), which may comprise one or more MAC-is Service Data Units (SDUs). Each MAC-is SDU is mapped to one or a part of a MAC-d PDU. The mapping of MAC-d PDU to RLC PDU is 1:1. To handle new transmission, at every TTI and for each logical channel the UE MAC-i/is generates a new TSN and associates it to a MAC-is PDU. At every TTI, the MAC-i can multiplex multiple logical channels, hence multiple MAC-is PDUs, each of which would carry its own TSN number.
Unfortunately, ambiguous conditions may exist when, illustratively, the RNC detects a nominal in-sequence TSN and an out-of-order TSN such as due to the re-acquisition of the common E-DCH resources by UE. If the RNC mistakenly delivers the out of order TSN to the next higher layer right away, this would induce out-of-order RLC PDU(s), which has a negative impact on capacity. The most common case is redundant RLC retransmission, such as might be triggered by the RLC upon detection of a RLC Sequence Number (SN) gap. Moreover, the delivering of an out-of-order Status RLC PDU could result in an RLC Reset condition.